Resolves to TODOs in the doorkeeper for helper

Moves token validation from DoorkeeperFor class into the Access Token
class.
Add specs for new methods in AccessToken and Scopes

adds test for includes_scope? acceptable?
Fix a style error in specs
Remove duplicated valid_token? method
Merge pull request #417 from edpaget/fix-todos-in-doorkeeper-for

Fix TODOs in the doorkeeper_for class